Process for the treatment of divided materials



Dec. 18, 1962 F. SCHAUB ETAL 3,

PRQCESS FOR THE TREATMENT OF DIVIDED MATERIALS Filed July 9, 1958 2Sheets-Sheet l INVENTORS FRANZ SCHAUB 8r BERNARD SCHLEPER Dec. 18, 1962F. SCHAUB ETAL PROCESS FOR THE TREATMENT OF DIVIDED MATERIALS 2Sheets-Sheet 2 Filed July 9, 1958 INVENTOR FRANZ SCHAUB AND BERNARDSCHLEPER ATTO EYS United States Patent Oilfice 3,958,584 Patented Dec.18, 1962 This invention relates to the treatment of divided materialswhich are entrained in a gas to elfect a transfer of materials betweenthe divided material and the gas.

In copending application Serial No. 572,559, filed March 19, 1956, thereis described a method for drying wet finely divided material whichinvolves dispersing the material in a gas and then passing the gasthrough an elongated chamber which is provided with a heating jacketsuitable for use in supplying heat to the entrainment or dispersing bytransfer through the chamber wall and thus .aiding in the drying of thefinely divided material. Ac-

cording to this method, the dispersion is passed axially through thechamber along a helical path.

It has been found that substantially improved results are realized whendrying divided material by the method of the aforementioned pendingapplication, by employing an amount of carrier gas for the dividedmaterial substantially equal to the amount necessary to satisfactorilyentrain the divided material. It has been further found, when operatingin this improved manner employing reduced amounts of carrier gas, agreat difference between the volume of inlet and the volume of outletgas exists when drying material having high moisture contents. Theconsiderable increase in volume of outlet gas is caused by the fact thata great volume of vapor is liberated from the solid particles beingdried. When the cross-sectional flow area along the helical path isconstant, the great increase in gas volume along the flow path, greatlyincreases flow velocity and consequently unnecessarily high pressurelosses occur. Moreover, we have found, that reduced drying capacitiesare incidental to the described conditions of great increase of volumealong the flow path.

According to the invention, these difficulties are overcome bymaintaining a substantially constant flow velocity of the gas along thehelical path.

The invention will be further described with reference to theaccompanying drawings, wherein specific embodiments of the invention aredepicted. In the drawings:

FIG. 1 is a schematic representation in elevation, of a device accordingto the invention;

FIG. '2 is a plan view of the device shown in FIG. 1;

FIG. 3 is a schematic representation, in elevation, of anotherembodiment of the invention;

FIG. 4 is a schematic representation, in elevation, of anotherembodiment of the invention, too, and

FIG. 5 is a schematic representation of the relation between thecross-sectional flow area and the volume of carrier gas.

In the various figures, like reference characters refer to correspondingparts.

In the embodiment shown in FIG. 1, the device of the invention comprisesan elongated chamber 6 having a heating jacket 7 which is provided withan inlet nozzle 8 and discharge nozzle 9. A center pipe 11 is disposedaxially within the elongated chamber in spaced relation ship therewithand provides annular space for the passage of the medium to be treated.Radially extending guide vanes 12 are disposed at spaced intervals alongthe center pipe 11, and serve to aid in defining a helical path throughthe annular space 10.

In operation of the device shown in FIG. 1, finely divided material isfed into the hopper 16 and is advanced through the pipe 22 by screwconveyer 17. A carrier gas, which can be air is drawn through pipe 18 byblower 19 and forced by blower 19 through heater 2t} and then on throughpipe 21 into the annular space 10 Where it is admixed, at 23, withfinely divided material and fluidizes the finely divided material. Theresulting dispersion passes into the annular space it) As can be bestseen in FIG 2, the feed pipe 22 directs the finely divided mate-. rialinto the annular space 10. The center pipe 11, and the arrangement ofthe radially extending vanes 12 cooperate to provide an axiallyextending helical flow path within the annular space 10, which thedispersion traverses. As the dispersion passes along this path, theparticles within the dispersion are thrown outwardly against the innerwalls of the elongated chamber 6 so as to form an upwardly movingcurtain along this wall. A heating medium is passed through the jacket 7and provides heat for drying of the particles passing along the helicalpath. From the elongated chamber 6, the dispersion passes to a separator27 or other treating zone (shown in FIG. 4).

Preferably, the amount of carrier gas employed to disperse the particlesof divided material is substantially equal to the amount required forsatisfactorily dispersing the divided material. It is not necessary touse a great excess of carrier gas. It is suificient for carrying out theprocess to use only so much gas as a carrier as is required to dispersethe material satisfactorily, the amount of gas being dependent upon thetype of material to be treated. The rapid evaporation of moistureresults in just as rapid an increase in the amount of gas. The smallamount of drying gas means a substantial economy with respect to boththe size of apparatus and the energy cost required. Despite itslimitation, this amount of gas is of such magnitude, that a change inmass flow rate through the elongated chamber due to evaporation from thewet material being treated, is very considerable, and, according to theinvention, the cross-sectional area of the helical flow path is alteredalong the path so that a substantially constant flow velocity existsthroughout the travel of the dispersion through the elongated chamber.

In the embodiment shown in FIG. 1, the guide vanes 12 are arranged sothat they aid in defining a helical path for the gas through the annularspace 10, and further, the angle of inclination of the guide means withrespect to the axis of the center pipe and elongated chamber, increasesalong the annular space in the direction of dispersion flow. Further,the center pipe 10 tapers in the direction of dispersion flow so thatits cross-sectional area along at least a portion of its lengthdiminishes in the direction of dispersion flow. Thus, the increasingangle of inclination of guide vanes and the change in crosssectionalarea of the center pipe aid in providing a con-- tinuously changingcross-sectional flow area for the dispersion. Preferably, this increasein flow area is such that it just compensates for the change in massflow along the flow area and maintains a constant flow velocity alongthe flow path.

As will be readily appreciated, various arrangements may be employed toobtain the desired variation in flow area along the helical fiow path.Thus, the desired variation can be obtained by varying the angle ofinclination of the guide vanes only without otherwise varyingcrosssections of elements of device. Alternatively, the crosssectionalarea of the center pipealone could be varied. Still another manner ofobtaining the desired constant velocity is to taper the elongated vesselin the direction opposite the direction of dispersion flow. Anycombination of these variations may be employed to obtain the desiredcontinuously changing cross-sectional area.

In FIG. 3, there is shown a device according to the ranged one above theother with the cross-sectional area of each section being larger thanthat of the preceding section. The cross-sectional area may be increasedin accordance with evaporation curve, i.e. with a considerable increasein the lower part and with only a moderate increase in the upper part.In general, it is assumed that about two thirds of the liquid evaporatesin the lower half and one third in the upper half. The increase incross-sectional area of the pipe sections is adapted to these conditionsand to the increase in gas volume. The increase in cross-sectional areaas dependent upon the increase in gas volume is shown in FIG. where a isthe pipe cross-sectional area, b the volume of the flowing 'gas phase inwhich the divided material is dispersed, the

material transfer being effected by transferred heat between thedispersion and its surroundings as the dispersion tnaverses a helicalpath.

The invention finds considerable usefulness in such processes whereinthe mass transfer is effected by heating the dispersion of dividedmaterial in gas phase as the dispersion passes through the transferzone, and the gas present in the dispersion and serving as a dispersionagent, is used in an amount substantially equal to merely the amountrequired to obtain satisfactory dispersion. It will be appreciated, thatthe proportion of gas required to obtain dispersion will depend on theparticular system, i.e. the composition of the gas and divided materialand operating condition including particle size of the divided material.i

EXAMPLE In producing polyethylene by the Ziegler process, a pulverulentproduct having a moisture content of about 100%, based on dry substance,is to be dried to a final moisture content. of 0.1%. According to theprocess of the invention, the solid material is introduced by a feedscrew into an elongated drying chamber, the walls of which are heated bya steam jacket. A center pipe is disposed axially within the dryingchamber and provided with guide vanes. For dispersing and feeding about200 kg./hr. of the moist material into the drying chamber, air at a rateof 100 m. /hr. is admitted through a pipe and imparted a helicalmovement by guide vanes, this .helical movement being brought aboutbelow the point where the moist powder is fed. At this point, the dryingchamber has a diameter of 110 mm. and the central pipe has a diameter of60 mm. The drying chamber has a length of 8 meters and its diameterincreases along this length to 200 mm, is. the cross-sectional flow areais about tripled corresponding to the increase in volume due to theamount of steam envolved during the drying process. The powder leavesthe drying chamber with a moisture content of about 0.1%. The pressuredrop for the flow through this conically widened elongated chamber isabout 180 mm. water. The steam consumption for driving out the moistureis 1.2 kg. of heating steam per kg. of evaporated water.

If a drying chamber having an invariable cross-sectional area is used, adiameter of 150 mm. and an amount of carrier air of about 200 m. /hr. isrequired for the same quantity of material to be dried and the pressuredrop is about 350 mm. water instead of 180 mm. Also, the specific heatconsumption is higher and is 1.4 kg. of heating steam per kg. ofevaporated water.

We claim: 7

1. Method for drying finely divided solid material which comprisesentraining the solid material in a heated gas stream, passing the gasstream with the entrained material upwardly through an elongated axiallyextending drying chamber of substantially circular cross-sectionalshape, guiding the gas through said chamber along a substantiallyhelical flow path while maintaining a substantially constant flowvelocity by increasing the volume of the flow path through the chamberin direct proportion to gas generated during the drying, said velocitybeing suflicient to throw the particles of solid material outwardlyagainst the chamber wall forming a moving curtain of particles along thewall, and externally heating the wall to maintain the same at the dryingtemperature for the material.

2. Apparatus for drying divided material comprising an elongatedvertically extending drying chamber of substantially circularcross-sectional shape, guide means positioned in said chamber defining avertically extending helical flow path of increasing cross sectionalflow area, corresponding to an increase in volume compensating for gasgeneration during drying to allow a substantially constant gas flowvelocity along said flow path, means for passing a gas stream withentrained solids to be dried through said flow path at a substantiallyconstant flow velocity sufiicient to throw the solids outwardly againstthe wall of said chamber, and means for externally heating the wall ofsaid chamber.

3. Apparatus, according to claim 2, in which said means for externallyheating the wall of said chamber comprises a heating jacket.

4. Apparatus, according to claim 2, in which said guide means comprisesguide vanes positioned within said chamher.

5. Apparatus, according to claim 2, including a pipe extending axiallythrough said chamber with guide vanes connected thereto defining saidhelical flow path, said pipe decreasing in cross-sectional size in anupward direction to form said flow path of increasing cross-sectionalflow area.

References Cited in the file of this patent UNITED STATES PATENTS

